The differentiation of T lymphocytes. IV. Net increases in low theta cells, maintenance of biological activity and death of high theta cells during short-term culture of mouse thymocytes.

Dissociated mouse thymocytes were cultured under optimized conditions in Marbrook vessels in order to follow some aspects of T-cell differentiation. Under particular conditions it was possible to obtain a 50% net increase in the total number of the peripheral T-cell like, minor low theta thymocyte subpopulation over the first day of culture, although this increase was lost with further incubation. The increase arose from proliferation of preexistent low theta cells,and not from transformation of the major, high theta population. Under all conditions the high theta cells died rapidly in culture. These results are in accordance with the view that the high theta and low theta thymocytes represent separate streams of T-cell development. There was a striking increase in responsiveness to phytohemagglutinin and concanavalin A after culture of thymocytes under these conditions. However, this was ascribed, not to the development of new immunocompetent T cells, but to selective cell death in the cultures eliminating some inhibitory elements. No increase in progenitors of cytotoxic lymphocytes was obtained. The results demonstrate some potential artifacts in assessing the immunocompetence of cultured thymocytes.